1. Field of the Invention
The present invention relates generally to photometric sampling techniques and, more particularly, is concerned with photometric sampling apparatus and method for extracting shape and reflectance properties of object surfaces.
2. Description of the Prior Art
Surface inspection is a vision application area that has great industrial significance. Inspection of machined parts, solder joint surfaces, and plastic sheets are just a few examples of industrial tasks that are laborious for humans to perform. Often, defects in the manufactured parts are too subtle to be detected by the human eye. Automated inspection systems promise to play an important and vital role in industry.
A typical inspection system first attempts to extract surface properties of the objects placed in its view. Subsequently, the system recognizes the objects on the basis of the extracted properties, and classifies them into end-application types. A key step in the process of surface inspection is extracting the surface shape.
Prior shape extraction techniques that are based on photometric measurements rely on assumed models of surface reflection. However, in many vision applications, the reflectance properties of object surfaces are not readily available. These prior techniques are unable to furnish information regarding the reflectance properties of the object surface without the aid of prior knowledge of the relative strengths of the Lambertian and specular components of surface reflection. While these techniques may be applied to specific tasks, they lack the ability to adapt to variations in the reflection properties of the inspected surfaces.
In many industrial applications, surface polish and surface roughness are found to be important inspection criteria. In such cases, surface reflectance properties may be interpreted as measures of surface polish and roughness. Further, reflectance properties may be used to segment an image in different regions; each region may be regarded as a different surface to aid the process of inspection. Thus, in actual practice, it can be expected that the extraction of surface shapes of varying reflectance characteristics will be required. As mentioned earlier, in use of the prior extraction techniques the reflection properties of the inspected surfaces must be known a-priori in order to determine surface shape. This common characteristic of all existing extraction methods is a significant drawback which considerably limits the versatility of inspection systems that are currently used in production lines.
Consequently, a need exists for an alternative approach to surface shape and reflectance extraction which overcomes the drawback of existing methods. It is highly desired to have a technique that can extract the shape of Lambertian, specular, and hybrid surfaces and, in addition thereto, also estimate the reflectance properties of each surface point.